The present invention relates to a pulverized coal combustion burner and a combustion method utilizing such a burner and, more particularly, to a combustion method using a pulverized coal combustion burner which pneumatically transfers and burns pulverized coal.
Hitherto, in this kind of pulverized coal combustion burners, occurrence of NOx during combustion is a large problem. Particularly, coal has a larger content of nitrogen, compared with gaseous fuel and liquid fuel. Therefore, it is more difficult to decrease NOx produced by combustion of pulverized coals than in the case of combustion of gaseous fuel or liquid fuel.
NOx produced by combustion of pulverized coal is almost all NOx that is produced by oxidizing nitrogen contained in coal, that is, so-called fuel NOx. In order to decrease the fuel NOx, various burner structures and combustion methods have been studied.
As one of the burning methods, there is a method of forming a low oxygen concentration zone within a flame and utilizing reducing reaction of NOx which becomes active when the oxygen concentration is low. For example, JP A 1-305206, JP A 3-211304, JP A 9-170714, JP A 3-110308, disclose methods of producing flame (reducing flame) of low oxygen concentration atmosphere and completely burning coal, and a structure having a fuel nozzle for pneumatically transferring coal at the center thereof and an air injecting nozzle arranged outside the fuel nozzle. In these methods, a low oxygen concentration zone is formed inside the flame, reducing reactions of NOx are progressed in the reducing flame zone, and an amount of NOx occurred within flame is suppressed.
Further, JP A 3-211304, JP A 9-170714 and JP A 3-110308 disclose formation of recirculating flows at a downstream side of the tip of a pulverized coal nozzle by providing a flame stabilizing ring or obstacle at the tip of the pulverized coal nozzle. That is, since a high temperature gas stays inside the recirculating flows, ignition of pulverized coals progresses and the stability of flame can be raised.
In general, since the ignitability of coal is not better than other fuel, it is difficult to raise the ignitability of the coal even if the above-mentioned various methods are adopted. Therefore, in combustion of coal, consumption of oxygen does not progress and a reducing zone is hard to be formed. In order to form a reducing zone, it is necessary to suppress mixing of fuel and air jetted from an air nozzle in the vicinity of the pulverized coal nozzle. The the mixing with fuel is suppressed by supplying the air to be supplied from the air nozzle in a swirling flow. However, when strong swirling is imparted to air, mixing of the air and fuel does not progress even at a downstream portion (more than three times as large as the diameter of a burner throat) separated from the burner due to centrifugal force, and it is hard to effect complete combustion. Therefore, in this kind of pulverized coal combustion, there is the problem that NOx occurs and unburned carbons are left in combustion ashes of pulverized coal.